Trump had spent his time in Brussels berating members of the military alliance for failing to spend enough of their money on defence, accusing Europe of freeloading off the USA and raising doubts about whether he would come to members' defence if they were attacked.

"The NATO leaders weren't really expecting this". They pay only a fraction of their cost.

In 2017, the alliance saw its biggest increase in defense spending "in a generation", he said, signaling to his aggrieved ally in the White House that "we see that all Allies have started to increase defense spending based on the national plans we have agreed to develop previous year". Though NATO allies have squabbled in the past, Trump seemed to be leaving in his wake not just bruised feelings, but deeper concerns about the future of the Western alliance that has served as a bedrock of the global order.

The alliance, which sees an attack on one member as an attack on all, was set up at the height of the Cold War, as a counterbalance to the Warsaw Pact between the Soviets and their satellite states. This is precisely why North Atlantic Treaty Organisation, for example, rallied behind the U.S. in the aftermath of the September 11 terrorist attacks and has sent many troops to fight and die in places like Afghanistan over the years.

Overall, the USA provides about 22% of this budget based off a formula that accounts for the national income of member states.

And while all member nations have agreed, in principle, to work toward the 2 percent spending goal, each country is ultimately beholden to unique political processes and must consult with their respective parliaments to secure increased military spending.

Nato's secretary-general said the main focus should be on all members reaching the current target of 2% of GDP. The target date to get military spending to 2% is 2024. "It confirms the goal of 2 percent by 2024".

"I think it's very sad when Germany makes a massive oil and gas deal with Russia, where you're supposed to be guarding against Russia, and Germany goes out and pays billions and billions of dollars a year to Russia", Trump told Stoltenberg earlier Wednesday.

Trump said he let countries know during this week's summit that he was "extremely unhappy". The US is paying for Europe's protection, then loses billions on Trade.

Amid the tumult, British Prime Minister Theresa May sounded a call for solidarity among the allies, saying, "As we engage Russian Federation we must do so from a position of unity and strength - holding out hope for a better future, but also clear and unwavering on where Russian Federation needs to change its behaviour for this to become a reality".

Trump declared that a joint natural gas pipeline venture with Moscow has left Angela Merkel's government "totally controlled" and "captive to Russian Federation". Not to mention, Trump already has a dubious relationship with Russian Federation at a time when much of the world, especially Europe, is concerned about its aggressive military activities.

Will Trump endorse Theresa May's leadership? "All the allies agree we have to make good on the commitments we made".

Asked about pressures on countries with weaker finances, he said, "We have many wealthy countries with us today but we have some that aren't so wealthy and they did ask me if they could buy the military equipment, and could I help them out, and we will help them out a little bit", he told a news conference. "I'm very strong on immigration".